The Bat has to die, but this can't be one of those boring, impersonal deaths, no, someone's gonna have to kill him. Who, of anyone, could take on the monumental task to kill Gotham's Dark Knight, not just in terms of abilities, but who could you really see killing Batman?
Any character from any media can be used (with limitations)
1) no one from Batman's regular rouge's gallery
2) no one above Superman in terms of power
They can have as much prep time as you could see them using, or who you think could just beat him in a random encounter.